Connection rod bearing

Rune

out riding my motarded wr426f 2001 two days ago, the engine stopped. Opening her up i dicovered that the lover bearing on the connection rod was broke.. an then the stupid question.. what could cause that to happend?? the bike has been run about 5000 km. (Regulary serviced) Is this a normal tear and wear condition that could happen now and then?? Anyone have a little info for this newbe from norway??